This invention relates to a method for making polyphosphoric acid.
Polyphosphoric acid is a phosphoric acid oligomer comprising molecules according to one or more of structural formulae (1) and (2):
PnO3n+1(n+2)xe2x88x92xe2x80x83xe2x80x83(1)
wherein n is a number greater than 1,
P2O5.x(O2xe2x88x92)xe2x80x83xe2x80x83(2)
wherein 0 less than x less than 1,
and typically having an average molecular weight of from about 110 to about 1,500 atomic weight units.
Polyphosphoric acid is made by dehydrating and polymerizing technical or food grade phosphoric acid, for example, in a packed column or by absorbing P2O5 vapors in a recirculating phosphoric acid stream.
Polyphosphoric acid is used in many diverse applications, including the manufacture of petroleum catalysts, surfactants for hand lotions and shampoos, oil well drilling compounds, industrial water treatment chemicals, flame retardant resins, super fibers and pigments, such as quinacridone pigments, and as a performance enhancing additive to petroleum products.
Some industrial uses of polyphosphoric acid generate by-product phosphoric acids that contain contaminants, such as phosphate esters, other organic compounds and metals. By-product phosphoric acids can also be generated by other industrial processes, such as, for example, processes for manufacturing polycarbonate resins, synthetic fibers, pharmaceutical intermediates and agricultural chemicals, as well as other industrial processes that use phosphorus containing materials. These by-product phosphoric acids can be neutralized and disposed of as a waste stream. Alternatively, such by-product phosphoric acids may be consumed by oxidizing the by-product acids in a thermal phosphoric acid production process, that is, production of phosphoric acids by combustion of elemental phosphorus in excess air, to thereby recover the phosphorus values from the by-product acids as phosphoric acid and to convert the organic contaminants to water and CO2.
The thermal phosphoric acid process is very costly due to the use of elemental phosphorus as its raw material and energy source. The production of polyphosphoric acid from thermal process phosphoric acid is correspondingly expensive, due to the high cost of the thermal process phosphoric acid raw material.
There is an interest in less costly ways to make polyphosphoric acid and to recover phosphorus values from by product phosphoric acid.
In a first aspect, the present invention is directed to a method for making polyphosphoric acid from phosphoric acid, comprising:
(a) providing a packed column, said packed column extending from a bottom end to a top end and having one or more first inlet openings located at or near the top end of the column and one or more second inlet openings located below the first inlet openings,
(b) introducing a first acid feed stream, said first acid stream comprising phosphoric acid, into the column at one or more of the first inlet openings;
(c) introducing a second acid feed stream, said second acid feed stream comprising phosphoric acid, into a stream of hot air to form a stream of hot air and acid;
(d) introducing the stream of hot air and acid into the column at one or more of the second inlet openings; and
(e) polymerizing phosphoric acid of the first and second acid feed streams to make polyphosphoric acid.
Introduction of the second acid stream into hot air stream allows dehydration and polymerization of the phosphoric acid into polyphosphoric acid to begin before the stream enters the column. The capacity of a packed column is limited by potential xe2x80x9cfloodingxe2x80x9d of the column. Flooding is a condition in which the gravity head of liquid in the column balances the gas pressure drop across the column and liquid can no longer flow through the packing at the desired feed rate. During operation of the column, the rate at which liquid feed streams are introduced to the column must be less than the rate that would cause flooding of the column. Compared to operation of a given packed column with a single acid feed stream fed into the top of the column, operation of the column according to the process of the present invention provides an increase in the capacity of the column. It is believed that the higher capacity is achieved, at least in part, because the process of the present invention allows more efficient use of the bottom section of the column and thereby allows, in total, a higher liquid feed rate to be maintained without flooding of the column.
In a second aspect, the present invention is directed to a method for consuming by-product phosphoric acid and reclaiming phosphorus values from the by-product phosphoric acid, said by-product phosphoric acid comprising phosphoric acid and one or more organic contaminants and said method comprising:
(a) providing a packed column, said packed column extending from a bottom end to a top end and having one or more first inlet openings located at or near the top end of the column and one or more second inlet openings located below the first inlet openings;
(b) introducing a first acid feed stream, said first acid feed stream comprising phosphoric acid, into the column at one or more of the first inlet openings;
(c) introducing a second acid feed stream, said second acid feed stream comprising by-product phosphoric acid, into a stream of hot air to form a stream of hot air and acid;
(d) introducing the stream of hot air and acid into the column at one or more of the second inlet openings; and
(e) polymerizing phosphoric acid of the first and second feed streams to make polyphosphoric acid, while simultaneously decomposing the one or more organic contaminants.
Consuming by-product acids according to the method of the present invention recovers the phosphorus value of the by-product acids by converting them to polyphosphoric acid, decomposes the organic contaminants, increases the production capacity of the polyphosphoric acid column and is much less costly than recycling the by-product acids in a thermal phosphoric acid production process, since natural gas is used as the energy source, rather than elemental phosphorus.